poolfan Probably 6mm ish, get a really good fitting hex key…You may have to remove the boiler for this, unless you can reach it through the access panel. Impact driver ideally, with many hits per second, if you don’t have one, tappy tappy. Don’t try and torque it off, if you want to use lots of torque, you will have to remove the boiler.
note only ever undo things on machines with the impact driver…never ever do anything up with it, even wheel nuts, undo with the driver and use a torque wrench to set em. Unless you got an expensive impact driver with Torque settings.
Clean up the plug and use enough PTFE so it’s tight all the way in, doing it up tight will make no difference, the PTFE should provide the seal.